linux-kernel - Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard policy

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Date:	Fri, 19 Feb 2016 16:06:10 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Yuki Shibuya <shibuya.yk@...s.nec.co.jp>
Subject: Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard
 policy



On 19/02/2016 16:04, Radim Krčmář wrote:
>> > 
>> > ... so in patch 7 concurrent _writes_ of reinject are protected by the
>> > lock, but reads are done outside it (in pit_timer_fn).  WDYT about
>> > making reinject an atomic_t?
> There was/is no harm in having reinject non-atomic.  This patch added
> notifiers, which is the reason for re-introducing a mutex.
> 
> Userspace can (and SHOULDN'T) call this function multiple times,
> concurrently, so the mutex prevents a situations where, e.g. only one
> notifier is registered in the end.

Yes, I understand why you added the mutex here; good catch indeed.  The
atomic_t is just to show that it's okay to read it outside the lock.
It's just for a bit of extra documentation.

Paolo